1. Field of the Invention
The present invention relates to a controller for a die cushion mechanism.
2. Description of the Related Art
In a pressing machine that performs press work, such as bending, drawing, and punching, there is known a die cushion mechanism that is provided as auxiliary equipment which adds a predetermined force (pressure), from a supporting member (called a bolster) for supporting a second mold to a movable-side supporting member (generally called a slide) for supporting a first mold to be used for press work, during a press operation. The die cushion mechanism is usually configured as follows. The die cushion mechanism directly or indirectly makes the slide (or the first mold), which is moving in a clamping direction, collide with a movable element (generally called a cushion pad) which is held with a predetermined pressure. The cushion-pad moves together with the slide, while applying force (pressure) to the slide until the slide is declamped after the clamping (molding). During this period, a peripheral area of a processing portion of a material to be processed is sandwiched between the cushion pad and the slide, thereby preventing the occurrence of wrinkles in the processed material.
In order to improve the precision of the press work using the die cushion mechanism, it is required to stably apply constant force (pressure) to the slide, during the period when the cushion pad moves together with the slide. However, most of conventional cushion mechanisms use a hydro-pneumatic power unit as a driving source. In this case, it has been difficult to control the force (pressure) on the slide, to a constant value, in response to a rapid variation of pressure due to an external factor when the slide collides with the cushion pad. In order to make it possible to perform force control with excellent response, a die cushion mechanism using a servo motor as a driving source has been developed in recent years, as disclosed in Japanese Unexamined Patent Publication No. 10-202327.
The die cushion mechanism described in Japanese Unexamined Patent Publication No. 10-202327 has a mechanism for lifting up and moving down a cushion pad disposed below a slide of a pressing machine, in response to the movement of the slide by the servo motor. During a period while the slide moves down (i.e., during a process operation), the slide applies a force to the cushion pad by colliding therewith. Then, the servo motor operates by force control, based on a predetermined force input instruction set corresponding to the position of the cushion pad, thereby adjusting the force (pressure) applied from the cushion pad to the slide while moving the cushion pad together with the slide. Collision and pressure are detected by detecting a load applied to an output shaft of the servo motor via the cushion pad.
According to the conventional die cushion mechanism using the servo motor driving described in Japanese Unexamined Patent Publication No. 10-202327, a set gain used for the control is constant during the press operation of the pressing machine.
However, mechanical characteristic of the die cushion mechanism changes according to the force (pressure) applied between the die cushion mechanism and the slide. Therefore, a physical gain of the force control changes. Consequently, when the force applied between the slide and the die cushion is large, the physical gain becomes large, and vibration easily occurs during the driving. On the other hand, when the force applied between the slide and the die cushion is small, controllability of the die cushion mechanism decreases.
The present invention has been made in the light of the above situation. It is an object of the present invention to provide a controller for a die cushion mechanism that can control the die cushion mechanism with high precision even when the mechanical characteristics changes.